The broad objective of this investigation is to define the mechanisms by which various hormones and vitamins regulate the metabolism and differentiation of bone cells, in order to determine their roles in modifying the structure and function of bone. We will utilize isolated viable cells dispersed from well differentiated bone, and therefore circumvent difficulties encountered in studying the metabolism of cells which are normally surrounded by a dense, often mineralized organic matrix. Use of isolated bone cells permits the application of biochemical techniques to the study of bone metabolism and differentiation which are not easily employed in studying intact bone. Since bone contains a heterogeneous population of cells, the availability of isolated cells provides the opportunity to separate this heterogeneous population into its constituent morphologically and functionally distinct cell types. The demonstration that bone cells retain the capacity to perform certain differentiated functions in vitro provides a starting point for investigating the hormonal and non-hormonal factors which influence bone cell differentiation. The sequence of metabolic events in hormone action which leads to modifications in cell function is to be studied, and these metabolic events will be correlated with alterations in bone cell morphology, as determined histologically. The study of isolated bone cells has already yielded considerable insight into the mechanism whereby ascorbic acid, adrenal glucocorticoids, insulin and parathyroid hormone affect bone cell metabolism. Results obtained thus far indicate that isolated bone cells are sensitive to physiological or near-physiological concentrations of hormones, a distinct advantage in studying hormone action. Further experiments with parathyroid hormone are now in progress, and additional experiments are planned to investigate the effects of other hormones known to influence bone as a tissue, including calcitonin, estrogens, androgens, vitamin D. These techniques will also be applied to the study of cells dispersed from human bone tumors, to determine whether these tumors are sensitive to hormones, information which could be significant in understanding the regulation of tumor growth, and in the treatment of patients with these tumors.